The optical effect of effect pigments is based on the directed reflection of light from light-refracting pigment particles which are predominantly two-dimensional in form and are oriented substantially parallel to one another. These pigment particles generally have a substantially transparent substrate and one or more coatings on the substrate. Depending on the composition of the coating or coatings on the pigment particles, interference, reflection and/or absorption phenomena produce impressions of color and lightness.
Irregularities in the substrate surface to be coated, or colored impurities in or on the substrate, may lead to unwanted scattered-light effects and hence to a reduced luster and also to instances of unclean color in the end product. Particularly when natural substrate materials are used, such as natural mica, these unwanted scattered-light effects and/or colored impurities arise.
The fundamental construction of pearlescent pigments and hence also of multilayer pearlescent pigments with an alternating arrangement of layers of high and low refractive index is known. Reference is made, exemplarily, to patent applications DE 41 41 069 A1, DE 43 19 669 A1, DE 196 14 637 A1, DE 198 02 234 A1, DE 198 08 657 A1, DE 198 22 046 A1, DE 199 07 313 A1, DE 199 41 253 A1, DE 199 53 655 A1 or DE 102 21 497 A1.
WO 2004/055119 A1 describes interference pigments based on coated, platelet-shaped substrates. The substrates in this case are covered with a first layer of SiO2, over which is applied, subsequently, a high-index layer, consisting for example of TiO2, ZrO2, SnO2, Cr2O3, Fe2O3 or Fe3O4, or an interference system comprising alternating high-index and low-index layers. The pigments may optionally also have an outer protective layer. In this way, silver-white interference pigments, or interference pigments with brilliant interference colors, are obtained, which are notable for performance properties, such as mechanical stability and photo stability, but which do not have a high gloss. The color of the interference pigments is not dependent or is only minimally dependent on the angle.
Thermally and mechanically stable metal oxide-coated effect pigments based on thin glass flakes with a thickness ≦1.0 μm are known from WO 2002/090448 A2. The effect pigments may be covered with one or more high-index and/or low-index layer(s). The glass flakes possess a softening temperature of ≧800° C.
Goniochromatic luster pigments are described in EP 0 753 545 B2. At least one layer stack comprising a colorless low-index coating and a reflecting, selectively or nonselectively absorbing coating, and also, optionally, an outer protective layer, is applied here to a multiply coated, high-index, nonmetallic, platelet-shaped substrate. The layer thickness of the low-index colorless coating reduces as the number of layer stacks applied to the substrate increases. The goniochromatic luster pigments exhibit an angle-dependent color change between two or more intense interference colors.
In accordance with WO 2004/067645 A2, a transparent substrate is coated with an uneven number—at least three—of layers of high and low refractive index in alternation. The difference in refractive index between the adjacent layers is at least 0.2. At least one of the layers differs in its optical thickness from the others. The resulting multilayer effect pigments therefore do not possess a layer construction in which the optical thickness of each layer is an uneven multiple of a quarter of the light wavelength for interference (no “quarter-wave-stack” construction).
Multilayer interference pigments with strong interference colors and/or with a strong angular dependency of the interference colors, consisting of a transparent base material coated with alternating layers of metal oxides of low and high refractive index, are described in EP 0 948 572 B1. The difference in the refractive indices is at least 0.1. The number and thickness of the layers are dependent on the desired effect and on the substrate used. Considering the construction TiO2—SiO2—TiO2 on a mica substrate, for example, pigments with a blue interference color are obtained when optically thin TiO2 and SiO2 layers with a layer thickness <100 nm are used, said pigments being more strongly colored than pure TiO2-mica pigments. The incidence of thick SiO2 layers with a layer thickness >100 nm produces pigments having a strongly pronounced angular dependency of the interference color.
The optical properties of effect pigments can be influenced, according to WO 2006/110359 A2, by a suitable particle size distribution. The glass flakes described here, classified and coated with a single metal oxide layer, have a D10 of at least 9.5 μm, preferably of 9.5 μm. A disadvantage is that the pigments have to have a size range with a D90 of not more than 85 μm, preferably of about 45 μm.
The prior art discloses various multilayer pearlescent pigments which possess appealing optical properties. Nevertheless, there continues to be a demand for improved products.